1. Field of Invention
The present invention relates generally to improvements in power trains of the type wherein the means for transmitting torque from the rotary output element of an internal combustion engine to a rotary driven element (such as the output shaft of a continuously variable transmission) comprises a torque monitoring sensor integrated with such transmission, in which a plate link chain including a plurality of rocker link elements is trained over a pair of adjustable pulleys or sheaves. More particularly, the present invention relates to improvements in the rocker elements, which interconnect the individual links of the plate link chain, to enhance the stability of frictional engagement of such a transmission over its entire operating range.
Continuously variable transmissions and torque monitoring sensors of the above outlined character are disclosed, for example, in commonly owned U.S. Pat. No. 5,879,253 entitled: "Torque Monitoring Apparatus", filed Nov. 26, 1997 and in commonly owned U.S. Pat. No. 5,725,447 entitled: "Power Train with Infinitely Variable Ratio Transmission ", filed Dec. 14, 1995 and are incorporated herein by this reference.
Such continuously variable transmissions typically include parallel pairs of axially movable pulleys or sheaves attached to the driven and output shafts of the transmission, which are actuated by hydraulic piston and cylinder assemblies. The torque sensor is designed to axially shift the adjustable sheaves into frictional engagement with the plate link chain, which is trained over a pair of such parallel sheaves, with a force depending upon the transmitted loads or upon the transmitted torque.
Each sheave or pulley is comprised of a pair of coneshaped discs or flanges wherein one flange is fixed to the driven and/or output shaft and the other is axially movable relative thereto. The friction surfaces of the flanges are generally conical, but vary from a true geometric cone profile to being slightly curved and/or quasi-spherical in axial cross-section to reduce wear between the plate link chain and the flanges. Further, it is advantageous in some instances to utilize asymmetrical sheaves comprised of confronting flanges with different quasi-sherical profiles to provide optimum wear and tracking characteristics.
The quasi-spherical profile of the flanges is useful in limiting the effects of so-called track misalignment or tracking error, that is, inclined running of the plate link chain that occurs due to the geometry of the flanges being forced into contact with the end surfaces of the rocker link elements in the plate link chain upon making a change in the transmission ratio during operation. The rocker link elements also function to interconnect the individual links of the plate link chain. The end faces of the rocker link elements may be curved and/or partially spherical and are designed to frictionally engage the contact surfaces of the flanges.
It has been shown that the quasi-spherical profile of the flanges and the curved end surfaces of the rocker link elements have different force transfer characteristics as the transmission ratio changes and, particularly when either of the two extreme transmission ratios in the operating range of the transmission are selected. At these operating extremes the rocker link elements transfer forces at their edges only. As a result a very high specific pressure will occur at the contact points between the rocker link elements and the flanges. In high performance transmissions locally excessive wear can result. Pitting at the contact surfaces of the rocker link elements may occur and the surfaces of the flanges will wear prematurely with the danger of hydrodynamic slippage.
Thus, the present invention has been developed to enhance the stability of frictional engagement of a continuously variable transmission in general and, more particularly, to reduce the edge transfer force and to limit the tracking error in the rocker link elements of the plate link chain, to improve the wear characteristics of the rocker link elements, and to overcome other shortcomings of the prior art.
2. Description of Related Prior Art
Continuously variable transmissions and torque sensors of the above outlined character and suitable for use with the present invention are disclosed, for example, in U.S. Pat. No. 5,725,447 to Friedman and in U.S. Pat. No. 5,879,253 to Friedman, et al. The torque sensors, which are disclosed in these prior patents, are designed to establish a clamping engagement between component parts of a torque transmitting apparatus depending upon the load or depending upon the transmitted torque. As a rule such torque sensors function to ensure a frictional engagement between the hydraulically adjustable, sheaves and the rocker link elements of the plate link chain, which are urged towards each other by a force depending upon the transmitted loads or the transmitted torque.
The application of an excessive force for urging these components together results in excessive wear whereas the application of an insufficient force entails slippage and also excessive wear between the sheaves and the rocker link elements of the plate link chain, which are maintained in frictional engagement with one another.
One example of a prior art plate link chain for a continuously variable transmission wherein the friction surfaces of the rocker link elements are essentially part spherical for cooperation with part spherical, cone-shaped surfaces of the such sheaves is disclosed in U.S. Pat. No. 5,792,013 (hereinafter '013 patent). With reference to the ('013) patent drawings, to prevent engagement of the edge regions of the rocker link elements (32, 33) with the respective cone-shaped flanges, contact surface lines (54, 55) located radially outwardly or inwardly, respectively, from a central surface line (49, 53) are inclined by an angle (.delta.) with respect to the central or base line. The angle of inclination (.delta.) increases, in positive or negative direction, respectively, starting at such central or base surface line about the intersection with the rocking surfaces (41, 42) in the direction of the cone-shaped surface (44) of the respective cone disk in engagement with the respective rocker link element.
However, this modification of the contour of the end facing friction surface (43) of the rocker link elements in the ('013) patent is provided only in the horizontal (i.e. circumferential) plane defined by the axis (46). In contrast, the present invention provides a substantially spherical or quasi-spherical end surface formed on the rocker link elements defined by two substantially perpendicular radii originating in both the horizontal (i.e. circumferential) and radial planes with respect to the axis of the rocker link element.